A Wrn "knockout" mouse (Wrn delta) was recently generated that eliminates WRN protein expression (similar to Werner Syndrome) but fails to impart a significant phenotype. This suggests that functional redundancies in mice mask the Wrn4 phenotype. The objective of this project is to characterize potential genetic modifiers of Wrn that affect genome instability and cancer in Wrn delta mice. The SPECIFIC AIMS are: 1. Determine the effect of genetic background on cancer development in Wrn delta mice. We will fast test the hypothesis that genetic background modifies the phenotype of Wrn delta mice. Our approach will be to introduce the Wrn delta allele into congenic inbred strains with different inherent cancer phenotypes, An extensive analysis of Wrn delta in different inbred strains will determine the relative contributions of genetic background and the Wrn delta allele to tissue-specific cancer risk. 2. Examine functional interactions of Wrn with Blm, Atm and Msh2. Studies in yeast show that parallel redundant pathways are often employed to preserve genome stability. A subset of these require the WRN homolog, SGS1. Accordingly, when sgs1 null mutations are combined with defects in parallel pathways, synergistic increases in spontaneous gross chromosomal rearrangements (GCRs) are observed. Using available knockout mouse strains, we will examine three mutants that are predicted to synergize with Wrn delta: Blm delta, Atm delta and Msh2 delta. 3. Determine the effect of DNA polymerase errors in Wrn delta mice. RecQ helicases are closely linked to replicative DNA polymerases (SGS1 is epistatic with DNA polymerase epsilon; WRN binds DNA polymerase delta), and it is hypothesized that spontaneous GCRs result, in large part, from DNA replication errors. This suggests that error-prone DNA replication wilt amplify the Wrn delta phenotype. We will test this idea by crossing Wrn delta mice with two "mutator" mouse lines recently generated in our laboratory that are defective for Pol delta or Pol epsilon proofreading. Together, these experiments will reveal Wrn functional interactions that affect genome instability and cancer in mice. Our long-term goal is to develop a meuse model that recapitulates fundamental aspects of the Werner Syndrome cancer phenotype, These studies will be integrated with the Cell Function-Monnat and Recombination Mechanisms-Maizels projects to develop a detailed molecular and organismal description of WNR function in rive. Our studies will also lay the ground work for a collaboration with the first project (Biochemistry-Loeb) evaluating Wrn Exo-, Hel- and SNP "knockin" mice.